ISTANBUL/BEIRUT, Feb 20 (Reuters) - An explosion believed to have been caused by a car bomb tore through a Syrian refugee camp at a border post on the frontier with Turkey on Thursday, killing five people, a monitoring group said.

Turkey is sheltering more than 600,000 refugees from Syria's almost three-year-long civil war and has kept its border open throughout the conflict.

Ambulances ferried the injured from the refugee camp to the southern Turkish city of Kilis, where a state hospital official said at least 40 people were being treated.

A Turkish border official said the blast near Turkey's Oncupinar border post, which sits opposite the Syrian Bab al-Salameh gate, could be felt several kilometres (miles) away, but that the border gate remained open.

Amateur video posted on the Internet showed what appeared to be three bodies under blankets. The Syrian Observatory for Human Rights monitoring group said five people were confirmed dead and a fire broke out in the camp after the blast.

Thousands of Syrians have been fleeing the city of Aleppo, some 60 km (40 miles) south of Kilis, in recent weeks because of a campaign of improvised "barrel bomb" attacks by President Bashar al-Assad's forces.

Many of the displaced live at a makeshift camp on the Syrian side of the border. Thursday's blast hit this area, according to video footage.

Towns near Bab al-Salameh have also seen sporadic clashes between the rebels fighting Assad and fighters from an al Qaeda splinter group, the Islamic State of Iraq and the Levant (ISIL).

PULLING OUT BODIES

Abu Osama, a camp administrator, said the explosion happened behind his office and at least 20 tents were reduced to rubble. "Some of the bodies and tents melted from the explosion," he said over the phone. "We have had thousands of new refugees come to this area the past 20 days because of the barrel bombing in Aleppo."

He blamed the attack on militants from ISIL and said it had fired several rockets near the camp over the past few days where members of the rival Islamic Front rebel group are based.

An eyewitness in the refugee camp who spoke on condition of anonymity said he pulled four bodies from the tents.

"The bomb exploded on the main road in front of the camp and affected a 500 metre radius. This road is frequently used by civilians, busses and trucks," he said. (Reporting by Ozge Ozbilgin and Seyhmus Cakan in Turkey, Oliver Holmes and Erika Solomon in Beirut, Editing by Mark Heinrich)